idway through Ben Stiller’s new comedy, “Tropic Thunder,” Robert Downey Jr. launches into a hilarious monologue detailing how to win an Oscar by playing someone that’s mentally challenged. As he scrolls through past winners like Dustin Hoffman, Tom Hanks and Peter Sellers, he’s careful to note that none of them ever went “full retard” in their roles. Apparently, doing so diminishes your chances of walking away with a golden statue, à la Sean Penn in “I Am Sam.” Satires, however, are the complete opposite. The more outrageous and ridiculous you are, the better chance you have of pulling things off, and though “Tropic Thunder” never gets so wild that it forgets about the story it’s trying to tell, it’s just wild enough to make the whole experience unforgettable.

The film takes place on the set of “Tropic Thunder,” a Vietnam War movie helmed by first-time director Damien Cockburn (Steve Coogan) and based on the book written by Vietnam Vet, Four Leaf Tayback (Nick Nolte). Dubbed as the most expensive war movie ever made, filming is halted only five days into production due to a skyrocketing budget and a cast of prima donnas that include A-list action star Tugg Speedman (Stiller), Australian method actor Kirk Lazarus (Downey Jr.), drug-addicted comedy star Jeff Portnoy (Jack Black), rapper-turned-actor Alpa Chino (Brandon T. Jackson), and rookie actor Kevin Sandusky (Jay Baruchel). Desperate to finish the film before foul-mouthed movie producer, Les Grossman (Tom Cruise), shuts it down, Cockburn and Tayback conceive a plan to take the actors out into the middle of the jungle and shoot the whole thing guerilla style. When Speedman is kidnapped by a Vietnamese drug cartel, however, the other actors must assume the roles of their characters in order to rescue him.

“Tropic Thunder” is one of those movies that either works or doesn’t. Spoofing the Hollywood system is already a tricky business – most of the time, these movies are so loaded with in-jokes that it’s difficult for the audience to relate – but when you couple that with the controversial casting of a white actor playing a black character for most of the film, there’s little room for error. Thankfully, Stiller pulls it off without a hitch, but it's mostly thanks to Robert Downey Jr.'s brave performance. His turn as the Oscar-winning Lazarus (who undergoes a controversial surgery to darken his skin in order to play the film’s African-American sergeant) is priceless. Underneath it all, he’s essentially playing a caricature of Russell Crowe (in a news segment, we discover Lazarus has anger issues) – a method actor who becomes so engrossed in his roles that he says things like “I don’t read scripts, scripts read me.”

The rest of the cast members also fare well, but they simply don’t compare to Downey Jr. Jack Black is surprisingly tolerable as the over-the-top broad comedy specialist, and Jay Baruchel is the glue that holds together the group (since he’s the only uncorrupted member of the quintet), but both Stiller and Jackson really struggle for laughs. Stiller, in particular, is probably the weakest of the bunch, and when his one-dimensional character is separated from the others, his scenes become even more tedious to sit through. It’s a good thing, then, that the movie contains so many great supporting roles, because it helps to keep things fresh throughout the film’s 105-minute runtime. Tom Cruise and Matthew McConaughey (as Speedman’s ass-kissing agent) are among the best of the bunch, and though both were only reported to have extended cameos, their characters are so important to the story that they practically are supporting roles. Up-and-comer Danny McBride also turns in a hilarious performance as the explosives-happy effects man, while Tobey Maguire’s pre-movie cameo (the film opens with fake trailers that introduce you to the three main actors) probably gets one of the biggest laughs.

It’s too bad that Cruise’s funny but overrated cameo is going to steal most of the limelight upon the film’s release, because “Tropic Thunder” really is Robert Downey Jr.’s show. Stiller may not be an ace director, but he’s a genius for recognizing that the actor could pull off such a seemingly impossible feat. Rick Baker deserves some credit for the amazing make-up effects, but without Downey Jr.’s willingness to completely immerse himself in the role, the character would have never worked quite as well as it does. It's ultimately the saving grace of the film, because even though “Tropic Thunder” is everything you’d expect it to be, it’s always funniest when the "Iron Man" star is involved. The movie does drag a bit towards the middle, but, much like the shootout at the end of “Hot Fuzz,” the grand finale reenergizes everyone involved. When all is said and done, “Tropic Thunder” probably won’t be the funniest comedy of the year, but it comes pretty darn close.

Unrated Director's Cut Blu-ray Review:

“Tropic Thunder” is one of those films that when you see it in theaters, you just know the DVD and Blu-ray is going to be packed with special features. And while that may be true of the single-disc release, the quality of the extras still isn't up to snuff. Along with a director’s cut of the film (which contains 14 minutes of additional footage), the first disc also includes two audio commentaries: a filmmaker track with director Ben Stiller, co-writer Justin Theroux and other various crew members, and a cast track with Stiller, Jack Black and Robert Downey Jr. – the latter of which records the entire commentary in character. Surprisingly, it’s the filmmaker track that proves to be more enjoyable, mostly because the idea of Downey Jr. speaking in character just doesn’t work when Stiller and Black flat out refuse to play along.

Disc two is where a majority of the extras can be found, including 18-minutes of deleted and extended scenes (as well as an alternate ending where Matthew McConaughey’s character doesn’t get away), four different production featurettes ranging from the making of the opening battle sequence (“The Hot LZ”) to special effects (“Blowing Shit Up”) and production design (“Designing the Thunder”), and a cast featurette made up almost entirely of behind-the-scenes antics. Rounding out the set is the faux documentary “Rain of Madness,” additional footage from that film (“Dispatches from the Edge of Madness”), a short make-up test with Tom Cruise, and the hilarious MTV Movie Awards sketch with Stiller, Black and Downey Jr.